Photodetectors such as photodiodes may be used for sensing ambient light or colors by placing a spectral filter with the desired sensing characteristics in front of the photodetector to select wavelengths of ambient light reaching the photodetector. In the context of this disclosure, ambient light may be understood to mean surrounding light from the environment that may include a mixture of wavelengths from the visible spectrum, and/or a range of electromagnetic waves of unspecified wavelengths. Traditionally, spectral filters have comprised unpatterned stacks of dielectric layers. The sensing characteristics of the spectral filters could be selected by adjusting the thickness of the layer or by adjusting the spectral filter material. However, these kinds of dielectric filters are costly and offer only one filter option per wafer whereby the filter is processed in an additional manufacturing step.
Recently, spectral filters formed from gratings and arrays have been reported. Subwavelength silicon gratings on quartz substrates as color filters have been reportedly demonstrated.
Furthermore, color filter arrays have been used in applications for image sensing, e.g. to select a particular wavelength of electromagnetic radiation for sensing by a photoconversion device. That is, the color sensitivity of the eye may be reproduced through the use of, for example, three different filters of an image, each configured to select a particular wavelength of electromagnetic radiation, wherein each of the filters behaves as a respective one of the three different color receptors in the eye. Current devices which may exploit the use of spectral filters for wavelength selection for photodetectors, have traditionally incorporated spectral filters merely as add-on external components to the photodetector. That is, the spectral filters are constructed independently from the photodector, in a separate manufacturing process. The separate spectral filters are then added on as an external component after the separate manufacturing process. The use of externally produced spectral filters in combination with existing photodetectors therefore represents a high cost to the manufacturing process. Furthermore, the process of incorporating independently constructed spectral filters with a photodetector may add further technical complexity to the manufacturing process. The spectral filter would further need to be designed to function with other electronic components within an integrated circuit.